The present invention relates to an apparatus for determining the diffuse reflectivity of a sample surface of small dimensions, especially the test field of a test strip for determining the substances contained in a body fluid, the apparatus having a semiconductor light emitter, preferably a light-emitting diode (LED), for the transmission of visible or infrared light to the sample surface, a measuring receiver for receiving the diffusely reflected light from the sample surface and for the production of an electrical signal corresponding thereto, evaluating means including electronic circuit means for converting the received signal to a value corresponding to the diffuse reflectivity, and a reference channel using the light of the same emitter for the calibration of the apparatus and for the elimination of errors of measurement which are caused by changes in the emitter or other electronic components. As used hereinafter and in the claims, the term "light" includes visible and infrared light.
Apparatus for determining the diffuse reflectivity of a surface are known in many different forms. However, apparatus which are intended to serve for determining the reflectivity of the test field of a test strip must satisfy very special requirements. It is precisely for apparatus of this kind, however, that there is an increasing need, because it has been possible for several years to manufacture test strips with such accuracy that the color change that takes place on them can be used, if precisely evaluated by the apparatus, for the quantitative determination of substances contained in body fluids, especially blood. Particularly important in this connection is the determination of blood sugar by means of test strips and corresponding evaluating apparatus whereby it has become possible for diabetics themselves to determine their blood sugar levels at relatively brief intervals. Consequently it has become possible to establish dosages of medicaments, especially insulin, more precisely than before, and consequently to reduce considerably the occurrence of the feared late consequences of diabetes.
Apparatus suitable for this purpose must satisfy very high accuracy requirements. On the other hand, they must achieve this accuracy under marginal conditions such as were not commonly encountered in the prior-art apparatus for determining reflectivity. In particular, only a very small area is available for measurement. For example, a test strip to be measured preferentially with the apparatus of the present invention has two directly adjoining test fields with a total surface measuring 6.times.6 mm, which are divided in the middle. Of the resultant test field width of 3 mm, only a width of about 1.2 mm is available for evaluation by reflection photometry.
In order to be made available to large numbers of users, the apparatus must be economical to produce. Other important requirements are compactness and battery operation. Considerable progress in this regard has been achieved by an apparatus which uses a semiconductor light emitter, especially a light-emitting diode (LED) whose light is used for the diffuse illumination of the sample surface through an Ulbricht sphere. The light diffusely reflected by the sample surface is detected by a light receiver through a special diaphragm design. Further details are described in the European patent application under Publication No. 75,766. With this apparatus, using a light-emitting diode as the light source, a precision can surprisingly be achieved such as formerly could be achieved only by using light sources of high constancy and intensity. It is particularly advantageous that the apparatus is in the form of a two-channel apparatus having a measuring channel and a reference channel. The term "reference channel." as used herein and in the claims, means an optical ray path which differs from the optical ray path of the measuring channel at least to the extent that it does not include the sample surface. Reference channels are known in a variety of forms in the photometric art. The reference channel of the above-mentioned apparatus is so designed that it is possible, after a single calibration procedure performed in the manufacture of the apparatus, to determine the diffuse reflectivity of the test fields of test strips without further calibration measurements.
Despite these very good results, the apparatus referred to above is not satisfactory in every respect. In particular, the Ulbricht sphere involves considerable manufacturing expense. Also, the amount of space which it requires is not available under all circumstances, especially when a plurality of closely adjacent test fields are to be evaluated on a so-called multiple-field test strip, without the need for moving them successively into the range of a single measuring apparatus. This would require several measuring instruments disposed closely side by side, which can be achieved only with very great difficulty if illumination through an Ulbricht sphere is used.
It is therefore an object of the present invention to make available an apparatus for determining the diffuse reflectivity of a sample surface which, while using a semiconductor light emitter as the source of radiation, will permit very precise measurement, plus compact construction and low cost of manufacture.